Electrical Box Fill Calculator (NEC 314.16)
Enter your box volume, conductor size, conductor count, devices, grounds, and clamps, and this tool computes the required fill volume against NEC 314.16(B) — instantly flagging PASS or OVER.
Electrical Box Fill Calculator
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Per-conductor volume allowance (NEC Table 314.16(B)): 14 AWG = 2.0, 12 AWG = 2.25, 10 AWG = 2.5, 8 AWG = 3.0, 6 AWG = 5.0 cu in. All devices count double their conductor allowance; all grounds together count once; all clamps together count once.
Simplified starting point for planning. Does not apply every NEC adjustment/derating factor. Verify against the current NEC and your local AHJ before installing.
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How box fill is calculated
Every item in an electrical box — conductors, devices, grounds, and clamps — uses up volume based on the largest conductor size present. Add them up and compare to the box’s rated cubic-inch volume.
| Step | Example: 12 AWG, 6 conductors + 1 device + grounds + clamps |
|---|---|
| Per-conductor allowance, 12 AWG | 2.25 cu in |
| 6 conductors | 6 × 2.25 = 13.5 cu in |
| 1 device (counts double) | 1 × 2 × 2.25 = 4.5 cu in |
| All grounds together (counts once) | 2.25 cu in |
| All clamps together (counts once) | 2.25 cu in |
| Total required | 13.5 + 4.5 + 2.25 + 2.25 = 22.5 cu in |
| Vs. an 18 cu in box | OVER by 4.5 cu in |
When the total exceeds the box volume, step up to a larger box (or a box extension) rather than crowding conductors — overfilled boxes make terminations harder and are a common inspection fail.
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Frequently asked questions
Why do devices count double the conductor allowance?
Each device (switch or receptacle) occupies space on both sides, so NEC 314.16(B) counts each strap at twice the volume allowance of the largest conductor connected to it.
Do all grounding conductors count as just one allowance?
Yes. Regardless of how many ground wires are in the box, the entire group counts once, at the volume allowance of the largest ground conductor present.
What happens if my box is over the limit?
An overfilled box is a common code violation and inspection fail — it makes safe terminations difficult and increases heat and damage risk. Move up to a larger box or add an approved box extension.
Is this box fill calculator free?
Yes — free, no sign-up. Enter your box volume, conductor size and count, devices, grounds, and clamps, and read PASS or OVER instantly. Estimates only; confirm against the current NEC and your local AHJ.